Error handling and exception mapping are crucial aspects of building robust and reliable APIs. They help ensure that your API provides meaningful error responses to consumers and improves the overall user experience. In this blog post, we will explore how to document API error handling and exception mapping using Java Spring REST Docs.

What is Java Spring REST Docs?

Java Spring REST Docs is a library that allows you to generate documentation for your RESTful APIs. It works in conjunction with the Spring MVC framework and provides an easy way to document request and response payloads, path parameters, query parameters, and more.

Error Handling in Spring MVC

In Spring MVC, error handling can be achieved using the @ControllerAdvice annotation, which allows you to define global error handling logic. To document error handling, you can start by creating a class annotated with @ControllerAdvice that handles exceptions and maps them to appropriate HTTP error responses.

@ControllerAdvice
public class GlobalExceptionHandler {

  @ExceptionHandler(NotFoundException.class)
  @ResponseStatus(HttpStatus.NOT_FOUND)
  public void handleNotFoundException() {
    // Exception handling logic
  }

  @ExceptionHandler(BadRequestException.class)
  @ResponseStatus(HttpStatus.BAD_REQUEST)
  public void handleBadRequestException() {
    // Exception handling logic
  }

  // Other exception handling methods
}

Documenting API Error Responses with Spring REST Docs

To document the error responses, you can use the MockMvc API provided by Spring MVC to perform API requests and capture the responses. Here is an example of how to document an error response using Spring REST Docs:

import static org.springframework.restdocs.mockmvc.MockMvcRestDocumentation.*;
import static org.springframework.restdocs.payload.PayloadDocumentation.*;

@Test
public void testCreateUserBadRequest() throws Exception {
  mockMvc.perform(post("/api/users")
    .contentType(MediaType.APPLICATION_JSON)
    .content("{ \"name\": \"John Doe\", \"age\": 15 }"))
    .andExpect(status().isBadRequest())
    .andDo(document("create-user-bad-request",
      responseFields(
        fieldWithPath("error").description("Error message"),
        fieldWithPath("timestamp").description("Timestamp of the error")
      )
    ));
}

In the above example, we are performing a POST request to create a user with invalid data and expecting a 400 Bad Request response. We use the document method from Spring REST Docs to generate documentation for this test case. The responseFields method is used to define the fields present in the error response.

Producing API Documentation

To generate the API documentation, you need to run your tests with Spring REST Docs enabled. Spring REST Docs will generate snippets containing the API request and response information. These snippets can then be combined to produce the final documentation.

By running the appropriate Gradle or Maven task, the snippets will be combined, and the API documentation will be generated in HTML or other formats of your choice.

Conclusion

Properly documenting API error handling and exception mapping is essential for building high-quality APIs. Java Spring REST Docs provides a convenient way to document API error responses and can be easily integrated into your existing Spring MVC project. By following the steps outlined in this blog post, you can ensure that your API documentation is comprehensive and up-to-date.

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